Despite some conservatives' concerns about passing immigration reform, a small shift in the Hispanic vote toward the GOP could yield that party a sizable number of additional House seats, a report says.

One of the arguments put forward by conservatives opposed to immigration reform is that it would do little to help the GOP win much in the way of Hispanic votes – but would provide Democrats with a huge new pool of eligible, left-leaning voters who could keep them in power for years to come.

The latest Republican to make this case is Rep. Lamar Smith of Texas, who writes in a Wednesday op-ed in Politico: "Immigration is the field Democrats want to lure Republicans to play on. Why? Because Democrats know they'll win. Democrats have done the math and realize that legalization inevitably would give them millions of votes, meaning more victories in congressional and presidential elections."

But in fact, there's compelling evidence that it may be Republicans who have the most to gain, politically, from immigration reform – while Democrats actually have little to gain and could wind up with sizable electoral losses. According to an analysis in The Georgetown Public Policy Review, even if Democrats had improved their margins among Hispanics in the 2012 election by double digits, it would have yielded them very little in the way of additional House seats. But a small shift in the Hispanic vote toward Republicans would have moved a significant number of seats into the GOP column.

Assuming a 42 percent turnout rate among Hispanics (an estimate, since no reliable data exist on Hispanic turnout), the report finds that a 10-percentage-point shift in Hispanic support toward Democrats would have netted that party only one additional House seat. A 20-point shift would have yielded Democrats only six new seats.

But for Republicans, it's an entirely different story: A five-percentage-point shift in the Hispanic vote toward the GOP would have given the party five additional House seats; a 10-point shift would have turned 12 seats to the GOP; and a 16-point shift would have given Republicans 21 additional seats.

There are few competitive congressional districts to begin with (already, looking ahead to 2014, analysts are estimating no more than 70 competitive districts – a number that will almost certainly come down as the election draws nearer). So a shift of five, 12, or 21 seats is nothing to sneeze at. Those numbers would drop somewhat with a lower turnout rate, but the overall trend – namely, that there are far greater opportunities for Republicans to make gains than for Democrats – remains the same.

As the report concludes: "These figures should put Democratic strategists on edge.... Democratic political operators must know that a bipartisan compromise on immigration reform might derail any attempt to retake the House if it allows the GOP to gain even a little ground with Hispanics."

Individual Republican lawmakers may still be driven more by fear of facing a primary opponent if they move to the center on immigration reform. But an analysis like this might prove a strong political incentive. If passage of an immigration bill leads to even small gains in support for the GOP among Hispanics, it could be the key factor that helps the party retain – or even expand – its majority in the House.